Two-dimensional crystals from light-harvesting complex I (LHC I) of the purple non-sulfur bacterium Rhodospirillum rubrum have been reconstituted from detergent-solubilized protein complexes. Frozen-hydrated samples have been analysed by electron microscopy. The crystals diffract beyond 8 A and a projection map was calculated to 8.5 A. The projection map shows 16 subunits in a 116 A diameter ring with a 68 A hole in the centre. These dimensions are sufficient to incorporate a reaction centre in vivo. Within each subunit, density for the alpha- and the beta-polypeptide chains is clearly resolved, and the density for the bacteriochlorophylls can be assigned. The experimentally determined structure contradicts models of the LHC I presented so far.